Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor canister, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to engine intake for combustion, further improving fuel economy.
However, engine run time in hybrid vehicles (HEVs) may be limited, thus limiting engine manifold vacuum, which is typically used to draw fresh air through the fuel vapor canister to desorb the stored fuel vapors. Thus, opportunities for purging fuel vapor from the canister may also be limited. Even if purge conditions are met, the conditions may only be held for a short period of time, leading to incomplete purge cycles. This may result in residual fuel vapors stored in the canister for long periods of time. Over the course of a diurnal cycle, the fuel vapors may desorb from the canister and result in increased bleed emissions.
The desorption of fuel vapors from adsorption material is an endothermic reaction. The desorption efficiency may be increased by heating the fuel vapor canister and/or the purge air. However, dedicated canister heaters add manufacturing costs, and provide an additional load on the vehicle battery. Further the adsorption of fuel vapor to adsorption material is an exothermic reaction. Increasing the efficiency of this reaction would require an additional canister cooling element. Heating the canister without subsequent cooling may limit fuel vapor adsorption in situations where a purge event is followed immediately by the venting of the fuel tank.
The inventors herein have recognized the above problems, and have developed systems and methods to at least partially address them. In one example, a system for an engine, comprising: a fuel vapor canister coupled to a fuel tank; a thermal jacket comprising a phase-change material, the thermal jacket spatially sheathing the fuel vapor canister; and an engine coolant passage positioned to transfer thermal energy between engine coolant and the phase-change material. In this way, the phase-change material may buffer the temperature of the fuel vapor canister by absorbing heat generated during hydrocarbon adsorption, and returning the heat to the vapor canister during hydrocarbon desorption. By coupling the phase-change material to engine coolant, the thermal capacity of the thermal jacket can be increased, as heated coolant can thus transfer thermal energy to the phase-change material to replace the thermal energy transferred to the canister during hydrocarbon desorption.
In another example, a method for a vehicle, comprising: circulating engine coolant through a thermal jacket comprising a phase-change material, the thermal jacket sheathing a fuel vapor canister; and then purging the fuel vapor canister to an engine intake. In this way, the fuel vapor canister may be heated prior to the purge operation, increasing the efficiency of the purge operation, thus decreasing the quantity of residual fuel vapor in the fuel vapor canister. In this way, bleed emissions may be reduced.
In yet another example, a system for a vehicle, comprising: a fuel tank coupled to a fuel vapor canister; an engine intake coupled to the fuel vapor canister via a canister purge valve; a vent line coupled between the fuel vapor canister and atmosphere via a canister vent valve; a thermal jacket configured to spatially sheath the fuel vapor canister, the thermal jacket comprising: a phase change material; an engine coolant inlet; an engine coolant outlet; and channels routed within the thermal jacket coupling the engine coolant inlet and the engine coolant outlet; and a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to: circulate engine coolant through the thermal jacket; and open the canister purge valve and the canister vent valve responsive to a temperature of the fuel vapor canister increasing above a temperature threshold. In this way, thermal energy from the engine coolant may be transferred to the phase change material, which in turn may transfer the thermal energy to the fuel vapor canister. This eliminates the need for an additional vapor canister heating element, thereby decreasing manufacturing costs and conserving energy within the engine system.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.